fonsinchen opened the ticket and wrote:

When refreshing the link stats for the next hops a vehicle will visit only one branch of each conditional order is refreshed. As the routing scheme may depend on all information about possible next hops of the vehicle to be available this is suboptimal. See http://wiki.openttdcoop.org/PublicServer:Archive_-_Games_261_-_270# gameid_262 for an example. As long as traffic was low on the "circle" lines the links frequently dropped as they weren't properly predicted. That lead to cargo only being routed along a prefix of the possible routes and consequently the vehicles only served that prefix and then went back to the source station because of the conditional "if empty go back" orders.

In order to fix that a few changes are necessary:
ulfhermann/openttd@dae6943
ulfhermann/openttd@4824ed8
ulfhermann/openttd@59eff23

The end result is that the order list is regarded as a tree and walked recursively to predict all possible ways a vehicle could go.

Reported version: trunk
Operating system: All

This issue was imported from FlySpray: https://bugs.openttd.org/task/5674